US-12617727-B2 - Method for producing zirconia sintered body

Abstract

A method for producing a zirconia sintered body includes: heating a zirconia molded body or a zirconia pre-sintered body, the heating includes a temperature increasing step, and a rate of temperature increase in a temperature region from a temperature at which the zirconia starts to shrink to a temperature at which the zirconia finishes shrinking in the temperature increasing step is adjusted to enable the zirconia molded body or the zirconia pre-sintered body to shrink at substantially a constant rate during temperature increase in each of zones of when the temperature region is evenly divided into a plurality of zones of specific temperature ranges.

Inventors

• Atsushi Matsuura
• Hiroyuki Sakamoto

Assignees

• KURARAY NORITAKE DENTAL INC.

Dates

Publication Date

20260505

Application Date

20201218

Priority Date

20191220

Claims (6)

1. 1 . A method for producing a zirconia sintered body, comprising: heating a zirconia molded body or a zirconia pre-sintered body, where the heating comprises a temperature increasing step, and a rate of temperature increase in a temperature region from a temperature at which the zirconia molded body or a zirconia pre-sintered body starts to shrink to a temperature at which the zirconia molded body or a zirconia pre-sintered body finishes shrinking in the temperature increasing step is adjusted to enable the zirconia molded body or the zirconia pre-sintered body to shrink at a substantially constant rate across the temperature region during the temperature increase in each of a plurality of zones of when the temperature region is evenly divided into the plurality of zones of specific temperature ranges, wherein the rate of temperature increase in each zone is set to 0.1 to 1.0 times an acceptable rate of temperature increase in each zone calculated from the formula (1) below, and a ratio of the rate of temperature increase in each zone to the acceptable rate of temperature increase in each zone is substantially the same, (Acceptable rate of temperature increase in each zone)=(maximum rate of temperature increase of furnace)/(coefficient of shrinkage of each zone) (1), wherein the coefficient of shrinkage of each zone in formula (1) is represented by the following formula (2), (Coefficient of shrinkage of each zone)=(shrinkage rate difference of each zone)/(temperature range/100) (2), wherein the formula (1) is solved by assuming that the coefficient of shrinkage of each zone is 1 when the coefficient of shrinkage of each zone calculated using formula (2) is less than 1, wherein the shrinkage rate difference of each zone is the difference between the shrinkage rate at the start temperature and end temperature of each zone.
2. 2 . The method according to claim 1 , wherein the specific temperature range of each zone is 50° C.
3. 3 . The method according to claim 1 , wherein the temperature region ranges from 1,050 to 1,400° C.
4. 4 . The method according to claim 1 , which has a total firing time of at most 120 minutes from a start of temperature increase in the temperature increasing step to an end of a retention period at the highest firing temperature of the temperature increasing step.
5. 5 . The method according to claim 1 , wherein the heating further includes a retention step of retaining the highest firing temperature of the temperature increasing step, following the temperature increasing step.
6. 6 . The method according to claim 1 , wherein the zirconia sintered body is suitable for dental use.

Description

TECHNICAL FIELD The present invention relates to a method for producing a zirconia sintered body. BACKGROUND ART For years, metal has been used for a range of dental products, including, for example, prostheses (such as veneer crowns, dental caps, crowns, and post crowns), orthodontic products, and products for dental implants. However, metals lack aesthetic quality because of the colors that are distinctively different from the color of natural teeth, and can cause allergic reaction when released from these products. These issues involving the use of metal have been addressed by dental products that use ceramic materials such as aluminum oxide (alumina) and zirconium oxide (zirconia) as alternative materials of metal. Particularly, zirconia excels in strength, and has relatively good aesthetics, and this, combined with the currently declining price of zirconia, has created a high demand for this material. In fabrication of a dental prosthesis with zirconia, a block unit or a disc-shaped work for milling (a material to be milled) that has been pre-sintered at a temperature about 400° C. to 700° C. below the temperature that produces an ideal sintered body is cut into a shape of a dental prosthesis with CAD/CAM equipment. The resulting workpiece of unsintered zirconia is then sintered by being held at a temperature as high as 1,400° C. to 1,650° C. The whole process from the start of temperature increase to the end of temperature decrease typically takes a total of 6 to 12 hours, including the retention period between temperature increase and temperature decrease. In response to a growing demand for short firing at the dental clinic, a furnace is proposed that enables firing in a short time period, as described in Patent Literature 1. However, such short firing involves deformation or distortion in fabrication of a large-sized prosthesis. Patent Literature 2, which relates to multilayer oxide ceramic bodies, particularly pre-sintered multilayer oxide ceramic blanks and oxide ceramic green bodies suited for dental applications, describes a pre-sintered multilayer oxide ceramic blank having a distortion coefficient (d=(HVmax−HVMIN)/HVAVE) of less than 0.4. While this enables reduction of prosthesis deformation, the technique is limited to a certain composition, and, even with this composition, involves deformation in large-sized prostheses (about twice as large as the 3 to 4 cm length described in Examples). CITATION LIST Patent Literature Patent Literature 1: JP 2015-531048 TPatent Literature 2: JP 2016-527017 T SUMMARY OF INVENTION Technical Problem It is accordingly an object of the present invention to provide a method of production of a zirconia sintered body that enables firing in a short time period, and that can reduce deformation even in large-sized prostheses, without limiting the composition. Solution to Problem The present inventors conducted intensive studies to find a solution to the foregoing problems, and found that the above issues can be solved with a short firing schedule created by adjusting the rate of temperature increase to specific rates. The present invention was completed after further studies based on this finding. Specifically, the present invention includes the following. [1] A method for producing a zirconia sintered body, comprising a heating step of heating a zirconia molded body or a zirconia pre-sintered body, wherein: the heating step includes a temperature increasing step, and a rate of temperature increase in a temperature region from a temperature at which the zirconia starts to shrink to a temperature at which the zirconia finishes shrinking in the temperature increasing step is adjusted to enable the zirconia molded body or the zirconia pre-sintered body to shrink at substantially a constant rate during temperature increase in each of zones of when the temperature region is evenly divided into a plurality of zones of specific temperature ranges. [2] The method according to [1], wherein the temperature range of each zone is 50° C. [3] The method according to [1] or [2], wherein the rate of temperature increase in each zone is set to be 0.1 to 1.0 times an acceptable rate of temperature increase in each zone calculated from the formula (1) below, and a ratio of the rate of temperature increase in each zone to the acceptable rate of temperature increase in each zone is substantially the same, (Acceptable rate of temperature increase in each zone)=(maximum rate of temperature increase of furnace)/(coefficient of shrinkage of each zone)  (1), wherein the coefficient of shrinkage of each zone in formula (1) is represented by the following formula (2), (Coefficient of shrinkage of each zone)=(shrinkage rate difference of each zone)/(temperature range/100)  (2), wherein the formula (1) is solved by assuming that the coefficient of shrinkage of each zone is 1 when the coefficient of shrinkage of each zone calculated using formula (2) is less than 1. [4] The method according